There have been remarkable developments in the functionality incorporated in the mobile phones. And mobile phones equipped with various functions such as a camera function and an image display function that uses a liquid crystal display of an excellent color resolution, in addition to the arrangement for the basic function to call are provided.
One of the mobile phones currently under development is capable of displaying images at 65536 (216) color resolution that are taken by using the camera function or that are acquired by means of communications.
In the case of a display at the 65536 color resolution, color data is represented by 16 bits. Such data is processed using a processor capable of performing arithmetic operations on a 16-bit basis and then is transferred via a 16-bit bus with most preferable efficiency.
Regardless of that high 65536 color resolution, there are certain images that cannot be displayed properly.
Examples for this are images of a wall or blue sky whose colors change very little, and hence, color unevenness is caused due to Mach band effects, resulting in the failure to attain a preferable image quality in terms of visual perception. In fact, a demand is heard from users for avoiding such color unevenness.
As an approach for improving the image quality, it can be suggested that an image is displayed as a rendition using the pseudo gradation based on the 65536 colors that can actually be displayed.
The pseudo gradation is a well-known technique in which pixels contained in a certain area of an image are broken down into small groups and are colored in multiple ways so that intermediate-level colors that are not actually displayed can be perceived in a pseudo way by an observer who is looking from a distance.
This pseudo gradation method is roughly divided into two categories, a dither method and a random dither method. A detailed description will be found in the non-patent literature 1, Chapter 8 “Image Gradation Conversion Method.”
If the number of colors that are represented in the pseudo way is increased using the pseudo gradation, it is expected that the color unevenness can be reduced or avoided while the efficiency in transferring image data remains a preferable level.
Conventional techniques introducing the most fundamental arrangements for realizing the pseudo gradation using the dither method are disclosed in the patent publications 1 and 2.
Patent publication 1: Japanese unexamined patent publication No. 6-125462
Patent publication 2: Japanese unexamined patent publication No. 6-250621
Non-patent literature 1: A Guide to Digital Image Processing, written by Hitoshi Kishi, published by CQ Publishing Company in 2002, 9th edition.